A variable voltage, variable frequency (VVVF) inverter rectifier driving an induction motor can be used to improve the travel comfort in one or two-speed elevator installations, offering a very cost-effective solution because of the low mechanical expense, since no speed sensors are required, which usually are installed to provide accuracy in approaching the floor level.
Such a control device for an elevator is described in U.S. Pat. No. 4,680,526, for example. In that control device, an inverter drives an induction motor. The primary current flowing into the induction motor is measured. Torque and slippage are determined on the basis of this primary current. In turn, output voltage and inverter frequency are controlled as a function of the slippage determined in this manner. In addition, the r.p.m. of the induction motor is estimated as a function of the determined torque current, and the induction motor's r.p.m. is controlled, so that the estimated r.p.m. signal corresponds to the nominal r.p.m.
The cost of such a control device is considerable. Furthermore, positioning contacts must be installed to provide accuracy when the elevator cabin approaches a floor level.
The invention is predicated on analysis of operation of induction motors and elevators driven thereby. For travel comfort, it is not always necessary to regulate the cabin speed in a closed loop so long as motor current is controlled so that the motor can produce the desired torque at any stator frequency. In this way, a ratio for torque and slippage is obtained, which can be approximately linear within a certain range. An example is shown in FIG. 1, where the slippage frequency is the abscissa and the electric torque is the ordinate.
The acceleration and braking requirements can be fulfilled with a suitable reference frequency profile. However, the elevator cabin speed depends on the load condition, because of the dependence of torque and slippage on the motor. For that reason, a reference profile, which ends in a creep speed section, is necessary for the level approach. FIG. 2 shows such a profile, wherein the abscissa is time and the ordinate is speed. The accuracy of the level approach depends on the creep speed, and a certain minimum travel time is required to ensure sufficient creep distance, even in the worst of cases.